1. Field of the Invention
Apparatuses and methods consistent with the present invention relate to a holographic light guide plate, and more particularly, to a holographic light guide plate with improved color dispersion, a manufacturing method and apparatus of the same, and an illumination unit for a display device using the holographic light guide plate.
2. Description of the Related Art
Non-emissive displays such as liquid crystal displays (LCDs) generally require a separate illumination apparatus such as a back light unit. FIG. 1 is an exemplary schematic view of a unit for a related art display device, with a holographic light guide plate. Referring to FIG. 1, a unit 10 of a related art display device includes a light guide plate 12 with a fine diffraction pattern formed on its upper or lower surface, a light source 11 disposed at a side of the light guide plate 12, and a diffusion plate 15 that evenly diffuses light that is emitted from the upper surface of the light guide plate 12.
Generally, a point light source such as a light emitting diode (LED) or a line light source such as a cold cathode fluorescent lamp (CCFL) may be used as a light source 11. The white light emitted from the light source 11 is incident on the light guide plate 12 made of a poly methyl meth acrylate (PMMA) with high light transmittance, for example. Such incident light proceeds along the inside of the light guide plate 12 through total reflection. Because a diffraction pattern 13 is formed on the upper surface of the light guide plate 12, a portion of the light incident on the upper surface of the light guide plate 12 is diffracted and emitted through the upper surface of the light guide plate 12 by means of the diffraction pattern 13. The light that is emitted through the upper surface of the light guide plate 12 is evenly diffused through a diffuser sheet 15, to illuminate a non-emissive display device, such as an LCD.
Methods of forming such diffraction patterns 13 include mechanically carving the pattern in the surface of the light guide plate 12, pressing a stamp with the diffraction pattern formed thereon against the surface where the diffraction pattern 13 is to be formed, and using laser beam interference. FIG. 2 illustrates a method of using laser beam interference to form a diffraction pattern 13 on a holographic light guide plate 12. Referring to FIG. 2, a photoresist 17 is applied on the surface of a light guide plate 12, and two parallel laser beams having the same wavelengths are simultaneously emitted to intersect on the photoresist 17. Interference between the two intersecting laser beams occurs, and the photoresist 17 is exposed in a diffraction pattern, and the exposed photoresist 17 is developed, forming a diffraction pattern in the same shape as the interference pattern on the light guide plate 12. Thereafter, light guide plates having the same diffraction pattern as that formed above are mass-produced through molding or injection molding methods.
FIG. 3 illustrates the shape of a diffraction pattern formed using a laser beam interference method, light distribution along a section thereof, and a Fourier transformation result of the light distribution. Referring to FIG. 3, as illustrated at the upper portion, the diffraction pattern formed on the entire surface of the light guide plate, has a uniform and single period distribution. In the section of FIG. 3 immediately therebelow, a related distribution of light has uniform peaks of light distribution corresponding to the diffraction pattern. The Fourier transformation illustrated at the bottom of FIG. 3 shows that the diffraction pattern has a single period distribution. The period of this diffraction pattern is determined by the angles (θ1 and θ2 in FIG. 2) and the wavelength λ of the two parallel laser beams, as illustrated in the following equation.
  Λ  =      λ          (                        sin          ⁢                                          ⁢                      θ            1                          +                  sin          ⁢                                          ⁢                      θ            2                              )      
However, because the refractive index and transmissivity of light vary according to its wavelength, color dispersion of white light occurs when it is transmitted to the upper surface of the light guide plate by the diffraction pattern. FIGS. 4A through 4C illustrate the above color dispersion, and show the exit angles for each of the red (R), green (G), and blue (B) colors according to the period of the diffraction pattern. Here, it is assumed that the refractive index of the light guide plate is n=1.59, the total reflection angle in the light guide plate is 39°, and the progressing angle of light at its center in the light guide plate is 64.5°. Here, when the wavelength of the red light is 620 nm, the wavelength of the green light is 530 nm, and the wavelength of the blue light is 460 nm, blue light, green light, and red light are emitted vertically when the period of the diffraction pattern is d=321 nm, d=369 nm, and d=453 nm, respectively. Accordingly, when a diffraction pattern having a single period is used, the emitted angle of light differs according to the wavelength of light, so that color dispersion results from the diffraction pattern.
In order to help solve this problem, a remedy using a combination of diffraction patterns with two or more different periods has been proposed. For example, when a combination of diffraction patterns with periods of 321 nm, 369 nm, and 453 nm are used, red, green, and blue light are combined in a vertical direction to produce white light. However, as illustrated in FIGS. 4A through 4C, the red, green, and blue light in non-vertical direction is not uniform, so that color dispersion still remains a problem.